The invention relates generally to two-way communication devices which must operate in an environment with electromagnetic interference. More specifically, the invention relates to external transceivers which remotely communicate with implanted medical devices. Part of this two-way communication link consists of control or programming signals transmitted from the external device to the implanted device for the purpose of altering the operation of the implanted device. The remainder of the link consists of low-level telemetry transmissions from the implant to the external device for the purpose of conveying information such as current status, battery level, or patient data.
Two-way communication with implanted medical devices imposes special problems which become even more acute in an interference-prone environment, especially where the medical devices are essential to maintaining life functions. Necessarily, implanted medical devices require ultra-low power levels from long-lived batteries. The most common implanted telemetry system employs a single, multi-turned coil to externally receive the low-level telemetry signals. These single-coil, receiving antennas are quite susceptible to interference from electric, magnetic and electromagnetic field sources which are present in the clinical environment.
Many implanted telemetry systems employ two separate external coils, one for transmission of control or programming information to the implanted device and the other to receive telemetry data from the implanted device. As a consequence, the tuning of the receiving and the transmitting circuits of the external transceiver are interdependent due to the close proximity of the two coils.
The present invention employs two noise-cancelling, antenna coils, which improve the signal-to-noise ratio significantly, and a circuit which permits transmission and reception of signals through the same antenna coil network without interactive tuning problems and without the employment of any switching devices. The improved, external transceiver circuit permits two-way communications with implanted medical devices in close proximity (on the order of 4 inches to 2 feet) to common interference sources such as cathode ray tubes and video monitors.